Abstract:

A fuel cell system which allows uniform fuel distribution to respective
fuel cells, comprising: a plurality of fuel cells 5 each including an
anode 2, a cathode 3 and an electrolyte membrane 4 disposed between the
anode 2 and the cathode 3; and a fuel supply flow path 6 branched to
supply fuel to each of the fuel cells 5. The sectional area of the fuel
supply flow path in the downstream of each branch connection is narrower
than that in the upstream. The above-described structure avoids the
decrease in the fuel supply pressure due to the reduced sectional area in
the downstream of the branch connection. Therefore, the fuel is supplied
to the respective fuel cell with uniform pressure.

Claims:

1. A fuel cell system comprising:a plurality of fuel cells including an
anode, a cathode and an electrolyte membrane disposed between said anode
and said cathode; anda fuel supply path branched to supply fuel to each
of said plurality of fuel cells,wherein said fuel supply path is disposed
so that a sectional area thereof in the downstream of a branch connection
is narrower than that in the upstream.

2. The fuel cell system according to claim 1, wherein said plurality of
fuel cells are arranged on the same plane.

3. The fuel cell system according to claim 1, wherein said plurality of
fuel cells each includes a fuel tank having an aperture on a top
thereof,wherein said anode is positioned over the aperture of said fuel
tank, andwherein said fuel tank is connected to communicate with said
fuel supply path.

4. The fuel cell system according to claim 3, wherein said fuel tank
receives fuel from said fuel supply path through an opening having a
diameter of 0.1 to 1.0 mm.

5. The fuel cell system according to claim 4, wherein said opening is
provided through a bottom face of said fuel tank.

6. The fuel cell system according to claim 3, wherein wicking material for
holding fuel is inserted into said fuel tank.

7. The fuel cell system according to claim 6, wherein each of said
plurality of fuel cells including a fuel supply control membrane which
selectively allows gas component of fuel to transmit therethrough,
andwherein said fuel supply control membrane is provided between said
fuel tank and said anode.

8. The fuel cell system according to claim 7, wherein said wicking
material is inserted so as not to cover said opening.

9. The fuel cell system according to claim 6, wherein said wicking
material is material which allows half or more of the fuel sent to said
fuel tank to flow on a surface thereof without being absorbed in the
wicking material.

10. The fuel cell system according to claim 1, wherein each of said
plurality of fuel cells includes an exhausting portion which exhausts gas
produced on said anode to outside.

11. The fuel cell system according to claim 10, wherein each of said
plurality of fuel cells includes a sealing member provided on a side of
said anode, andwherein said exhausting portion is an exhausting path
formed through said sealing member.

12. The fuel cell system according to claim 10, wherein said sealing
member is provided with at least one penetrating hole which penetrates
said sealing member in an up-and-down direction,wherein a member disposed
on said sealing member has a hole at a position corresponding to said
penetrating hole, andwherein said exhausting portion allows said anode to
communicate with outside through said penetrating hole and said hole.

13. The fuel cell system according to claim 1, wherein each of said
plurality of fuel cells includes:a vaporization suppressing member
disposed on said cathode and having a moisturizing property; anda meshed
cover member disposed on said vaporization suppressing member.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a fuel cell system, more
particularly, to a fuel cell system in which a plurality of fuel cells
are arranged in a planar stack structure.

BACKGROUND ART

[0002]Fuel cells incorporating a membrane and electrode assembly,
(hereafter referred to as the MEA) are known in the art in which an
electrolyte membrane is supported between an anode and a cathode.

[0003]Among such fuel cells, a type of fuel cell which directly supplies
the liquid fuel to the anode is referred to as the direct fuel cell. In
the direct fuel cell, the supplied liquid fuel is decomposed on the
catalyst supported on the anode to produce protons, electrons and
intermediate products. The produced protons travel to the cathode through
the electrolyte membrane. Also, the generated electrons travel through an
external load to the cathode. On the cathode, the protons and the
electrons react with oxygen in air to produce reaction products. This
results in electric power generation.

[0004]For example, in a direct methanol fuel cell (hereafter, referred to
as the DMFC), which uses methanol aqueous solution as the liquid fuel,
the reaction represented by the following Reaction Formula 1 occurs on
the anode, and the reaction represented by the following Reaction Formula
2 occurs on the cathode:

CH3OH+H2O→CO2+6H++6e-, (Reaction Formula
1)

and

6H++6e-+ 3/2O2→3H2O. (Reaction Formula 2)

[0005]Solid polymer electrolyte fuel cells that use liquid fuel are now
expected as power sources for various electronic devices, such as
portable devices, due to the easiness of size and weight reduction. For
example, a plurality of fuel cells may be connected for the use as a
power source of a device requiring relatively high power output, such as
a personal computer (PC) and the like, because of the low power output of
a single MEA (hereafter, each minimum unit configuring a stack is
referred to as the fuel cell). As a structure incorporating multiple fuel
cells, there are known: a bipolar structure in which unit cells of fuel
cells are stacked in the thickness direction of the cells; and a planar
stack structure in which unit cells of fuel cells are arrayed in plane.

[0006]The planner stack structure is more advantageous for devices
designed to fulfill the portability requirement, such as notebook PCs,
due to the limitation of the thickness. For the planar stack structure,
two types of systems are known for supplying fuel to each of the
plurality of fuel cells: the serial flow system in which the fuel is
supplied to the respective fuel cells in series, and the parallel flow
system in which the fuel is supplied to the respective fuel cells through
branches from the central flow path.

[0007]In the serial flow path system, the fuel having been used for
electric power generation in the upstream cells is also used in the
downstream cells, and therefore the downstream cells experience thin fuel
concentration and high temperature. As a result, the electric power
generation conditions may be largely different between the upstream fuel
cells and the downstream fuel cells. Electric power generation with the
same current in different electric power generation conditions may cause
the downstream cells to be subjected to a severe electric power
generation environment. This causes the downstream fuel cells to suffer
from enhanced deterioration.

[0008]On the contrary, the parallel flow path system achieves uniform fuel
distribution and thereby allows stable electric power generation, because
of the ability of supplying the fuel to respective fuel cells with the
same concentration and temperature.

[0009]However, the parallel flow path system requires branching and
joining of the fuel in the supply flow path and in the return flow path.
Since the inner pressure of the flow path changes at the branching points
and joining points, it is difficult to uniformly distribute the fuel to
the respective fuel cells. Additionally, the supply to the respective
fuel cells may be off-balanced because of the local deviation in the
pressure distribution, since the fuel circulating system and the like
operates as a liquid-vapor mixture system due to the CO2 produced by
the electric power generation and introduced into the fuel cells and the
return flow path.

[0010]Therefore, it is desired to provide a technique for uniformly
distributing the fuel to the respective fuel cells for a planar stack
type fuel cell system with the parallel flow path system. It is also
desired to provide a technique for avoiding the CO2 produced by the
electric power generation being introduced into the fuel cells and the
return flow path.

[0011]Various approaches have been proposed for uniformly distributing the
fuel in the fuel cell system with multiple fuel cells. For example, in
the fuel cell described in Japanese Laid Open Patent Application No. JP-A
2003-203647, an approach has been proposed in which grooves and holes are
provided for a separator and the liquid fuel is supplied through these
flow paths.

[0012]Also, in the fuel cell described in Japanese Laid Open Patent
Application No. JP-A 2002-175817, an approach has been proposed in which
a CO2 exhaust groove is provided within the fuel cell to separate
the exhausted CO2 from the supplied fuel.

[0013]Japanese Laid Open Patent Application No. JP-A 2002-56856 discloses
a fuel cell in which a fuel supply flow path and a CO2 exhaust
groove are formed on an interface between an electrolyte membrane and a
catalyst layer.

[0014]Also, Japanese Patent Gazette No. 3442688 and Japanese Laid Open
Patent Application No. JP-A 2001-15130 disclose a fuel cell configured to
decrease the fuel vaporization through the MEA by supplying the liquid
fuel to the anode after the evaporation through the fuel supply layer.

[0015]Also, Japanese Laid Open Patent Application No. JP-A 2001-102070
discloses a fuel cell characterized in comprising: an electrolyte
membrane; fuel and oxidant electrodes opposed to each other across the
electrolyte membrane; a fuel container for holding the liquid fuel on the
fuel electrode surface; and a separation membrane formed in the fuel
container to separate carbon dioxide gas and the liquid fuel and to
selectively exhaust the carbon dioxide gas generated from the fuel
electrode out of the fuel container.

[0016]Also, Japanese Laid Open Patent Application (JP-P 2003-317745A)
discloses a direct methanol fuel cell pack of the spontaneous respiration
type, which includes: an electrolyte membrane; a membrane electrode
assembly (MEA) in which a large number of unit cells are formed with a
large number of anode electrodes provided on a first plane of the
electrolyte membrane, and with a large number of cathode electrodes
provided on a second plane of the electrolyte membrane on the side
opposite to the first plane to be associated with the respective anode
electrodes; a fuel supply room storing therein the fuel to be supplied to
the anode electrodes, and attached with a fuel supply plate through which
a large number of fuel supply holes are formed to pass the inner fuel;
and a wicking sheet provided in the shape of a fuel supply path between
the fuel supply plate and the MEA to diffuse and supply the fuel to the
anode electrodes of the MEA through the fuel supply plate.

[0017]Also, Japanese Laid Open Patent Application No. JP-A 2003-346862
discloses a fuel cell including: a positive electrode for reducing
oxygen; a negative electrode for oxidizing a fuel; an electrolyte layer
formed between the positive electrode and the negative electrode; and an
exhaust port for exhausting the fuel and the substances generated when
the fuel is oxidized, the fuel cell being characterized by at least one
approach selected from a group consisting of: an approach in which a
catalyst is provided for the exhaust port for oxidizing the substance
generated by the imperfect fuel oxidation; and an approach in which an
absorbent is provided for the exhaust port for absorbing the substance
generated by the imperfect fuel oxidation.

[0018]Also, Japanese Laid Open Patent Application No. JP-A 2002-280016
discloses a fuel cell for smoothly exhausting the carbon dioxide, which
is reaction by-product, through a gas exhaust path provided in a current
collector.

[0019]Japanese Laid Open Patent Application No. JP-A 2001-283892 discloses
a single electrode cell pack for a fuel cell that contains: cells each
having a membrane arranged in a center portion, a cathode and anode which
are arranged on the respective sides of the membrane; an electricity
collector electrically connected with the cathode and the anode; and an
electric connection member providing electrical connections among the
cells, characterized in that the number of the cells is two or more, and
the cells are commonly arranged on a specific flat surface across a
cavity in which the electric connection member is arranged, wherein the
single electrode cell pack is provided with: a porous fuel diffusion
member provided in contact with the anode to allowing the fuel to be
diffused into the cell; a porous fuel contact member provided in contact
with the anode to bring the fuel and air into contact with each other
inside the cell; anode and cathode end plates arranged on the anode side
and the cathode side of the cell, respectively, in order to protect the
cell; fuel supplying and exhausting means for supplying the fuel to the
portion of the cavity on the anode side and exhausting the fuel; a fuel
flow stop member for preventing the fuel circulating in the cavity on the
anode side from flowing into the portion on the cathode side from the
cavity, in the portion on the cathode side of the cavity; and a sealing
portion for sealing an anode portion in which the anode is arranged in
the cell, and the cavity corresponding to the anode portion, from
outside.

[0020]Nevertheless, none of the above-mentioned patent documents describes
a technique for uniformly distributing the fuel to the respective fuel
cells. This requirement is desirably fulfilled with a simple structure in
an aspect of avoiding the CO2 produced by the electric power
generation being introduced into the fuel cell and the return flow path.
It is also desired to provide a technique for avoid the CO2 being
introduced into the fuel cell and the return flow path after a long-time
operation of the fuel cell.

DISCLOSURE OF INVENTION

[0021]An object of the present invention is to provide a fuel cell system
and fuel cells which allow uniformly distributing the fuel to the
respective fuel cells.

[0022]Another object of the present invention is to provide a fuel cell
system and fuel cells for avoiding the CO2 produced by the electric
power generation being introduced the fuel cells and the return flow
path.

[0023]Still another object of the present invention is to provide a fuel
cell system and fuel cells for avoiding the CO2 produced by the
electric power generation being introduced into the fuel cell and the
return flow path after a long-time operation of the fuel cells.

[0024]In order to address the above-mentioned objects, the fuel cell
system according to the present invention is provided with: a plurality
of fuel cells each including an anode, a cathode and an electrolyte
membrane disposed between the anode and the cathode; and a fuel supply
flow path branched to supply fuel to each of the fuel cells. The fuel
supply flow path has at least one branch connection at each of which the
sectional area in the downstream is narrower than that in the upstream.

[0025]The flow quantity of the fuel flowing through the flow path is
decreased in the downstream of the branch connection. The decrease in the
fuel flow quantity causes the decrease in the flow pressure in the
downstream, when the sectional area of the fuel supply flow path is equal
between the upstream and the downstream of the branch connection. The
above-described structure, in which the sectional area of the flow path
on the downstream of the branch connection is decreased, avoids the fuel
flow pressure drop. This allows fuel supply to each fuel cell with a
uniform pressure.

[0026]In this fuel cell system, the fuel cells are preferably arranged on
a plate-shaped frame so as not to overlap each other. Each fuel cell has
a fuel tank for accumulating the fuel supplied from the fuel supply flow
path.

[0027]In this fuel cell system, the fuel tank preferably receives the fuel
from the fuel supply flow path through an opening having a diameter of
0.1 to 1.0 mm.

[0028]Supplying the fuel to the fuel tank through the opening having a
diameter of 0.1 to 1.0 mm in this way is preferable from the viewpoint
that the fuel flows in only one direction to the fuel tank from the fuel
supply flow path. The supply of the fuel to the fuel tank from the fuel
supply flow path tends to be insufficient when the diameter of the
opening is 0.1 mm or less. When the diameter of the opening is 1.0 mm or
more, on the other hand, the flow of the fuel inside the fuel tank tends
to be non-uniform.

[0029]In the above-described fuel cell system, the opening is preferably
to be provided through the bottom face of the fuel tank. Providing the
opening through the bottom face of the fuel tank promotes the one-way
fuel flow from the fuel supply flow path to the fuel tank; this avoids
the backflow from the fuel tank to the fuel supply flow path. The
variation in the inner pressure of the fuel supply flow path is further
suppressed by avoiding the backflow to the fuel supply flow path. As a
result, the fuel is supplied to the respective fuel cells with more
uniform pressures.

[0030]In this fuel cell system, a wicking member for holding the fuel is
preferably inserted into the fuel tank. Here, the wicking member has a
function of holding the fuel flowing into the fuel tank. The
thus-described insertion of the wicking member disperses the flow
direction of the fuel flowing into the fuel tank, thereby attaining the
uniformity.

[0031]In this fuel cell system, each fuel cell preferably includes a fuel
supply control membrane formed on the fuel tank which selectively
transmits only gas component of the fuel to supply to the anode.

[0032]The provision of the fuel supply control membrane as mentioned above
allows the fuel to be supplied to the anode with an optimal flow
quantity.

[0033]In this fuel cell system, the wicking member is preferably inserted
into the fuel tank so as not to cover the opening.

[0034]The wicking member provided so as not to cover the opening as
mentioned above allows the fuel flowing into the fuel tank from the
opening to mainly flow on the surface of the upper portion of the wicking
member (the portion of the wicking member positioned near the fuel supply
control membrane). Such structure allows the fuel to be stably supplied
to the anode inside the fuel tank. This enhances selective supply of the
liquid fuel to the surface portion of the wicking member, which is
located near the anode, thereby promoting the vaporized fuel supply
through the fuel supply control membrane.

[0035]Also, the wicking member is preferably formed of material which
allows half or more of the fuel sent to the fuel tank to flow on the
surface thereof without being absorbed by the wicking member; it is more
preferable that the wicking member is formed of material which allows 70%
or more of the fuel sent to the fuel tank to flow on the surface thereof.

[0036]The structure in which half or more of the fuel sent to the fuel
tank flows on the surface of the wicking member helps to supply the fuel
through the fuel supply control membrane to the anode. When the amount of
the fuel flowing on the surface of the wicking member is half or less, on
the other hand, the fuel is mainly held by the wicking member, making it
difficult for the fuel to flow through the upper portion of the wicking
member. That is, the fuel is difficult to be supplied through the fuel
supply control membrane to the anode. The use of material as the wicking
member which causes the increase in the flow path resistance after
absorbing the fuel and thereby allows most of the supplied liquid fuel to
flow on the surface of the wicking member determines the directivity of
the fuel flow, achieving sufficient fuel supply to the anode 2 through
the fuel supply control membrane even when the electric power generation
is executed with a higher current condition.

[0037]In the fuel cell system according to the present invention, each
fuel cell is preferably provided with an exhausting portion for
exhausting the gas produced on the anode to the outside.

[0038]When CO2 produced by the electrode reaction on the anode is
exhausted to the outside, the CO2 is not accumulated between the
anode and the fuel supply control membrane. This avoids the supply of the
vaporized fuel from the fuel supply control membrane to the anode being
disturbed, since the pressure between the anode and the fuel supply
membrane is not increased. Therefore, the fuel is stably supplied to the
anode.

[0039]In this fuel cell system, it is preferable that each fuel cell
includes a sealing member for sealing the side of the anode from the
outside, and the exhausting portion is an exhausting path provided for
the sealing member so as to exhaust the gas from the anode to the
outside.

[0040]Such structure avoids the accumulation of CO2 between the anode
and the fuel supply control membrane, since the CO2 produced by the
electrode reaction on the anode is exhausted to the outside. The supply
of the vaporized fuel from the fuel supply control membrane to the anode
is not disturbed, since the pressure between the anode and the fuel
supply membrane is not increased. As a result, the fuel is stably
supplied to the anode.

[0041]In the fuel cell system according to the present invention, it is
preferable that at least one penetrating hole is provided through the
sealing member, and a hole is provided for a member disposed on the
sealing member at the position corresponding to the penetrating hole, and
that the exhaust portion allows the communication between an anode (2)
and the outside through the penetrating hole and the hole.

[0042]Such structure avoids the accumulation of CO2 between the anode
and the fuel supply control membrane, exhausting the CO2 produced by
the electrode reaction on the anode to the outside. The supply of the
vaporized fuel from the fuel supply control membrane to the anode is not
disturbed, since the pressure between the anode and the fuel supply
membrane is not increased. Therefore, the fuel is stably supplied to the
anode. Additionally, the gas component is not introduced into the fuel
circulating system, and this avoids the rapid variations in the fuel
supply and in the flow path resistance inside the fuel cell, which are
caused by the deposition of the CO2 in the form of babbles on the
surface of and inside the wicking member.

[0043]In the above-described fuel cell system, each fuel cell is
preferably provided with: an evaporation suppression member arranged on
the cathode and having a moisturizing property; and a meshed cover member
arranged on the vaporization suppressing member. The movement of the
protons produced on the anode to the cathode through the electrolyte
membrane occurs under the existence of water. The vaporization
suppressing member and the cover member suppress the vaporization of the
water produced on the cathode in the electric power generation. This
avoids the water vaporization from the electrolyte membrane (4).

[0044]The present invention provides a fuel cell system and fuel cells
which allow uniform distribution of the fuel to the respective fuel
cells.

[0045]Additionally, the present invention provides a fuel cell system and
fuel cells which avoid CO2 produced the electric power generation
being introduced into the fuel cell and the return flow path.

[0046]Furthermore, the present invention provides a fuel cell system and
fuel cells which avoid CO2 produced the electric power generation
being introduced into the fuel cell and the return flow path even after a
long-time operation of the fuel cells.

BRIEF DESCRIPTION OF DRAWINGS

[0047]FIG. 1 is a schematic view showing one example of the fuel flow path
structure in a fuel cell system 1 according to the present invention;

[0048]FIG. 2 is a sectional view of the fuel cell structure on the B-B'
section shown in FIG. 1 of the fuel cell system 1 according to a first
exemplary embodiment;

[0049]FIG. 3 is a perspective view depicting the structure of a fuel tank
8 within the fuel cell system 1 according to the first exemplary
embodiment;

[0050]FIG. 4 is an exploded perspective view depicting the configuration
of the portion positioned between an anode electric collector 24 and a
cathode electric collector 25 in the fuel cell system 1 according to the
first exemplary embodiment;

[0051]FIG. 5 is a sectional view of the fuel cell structure on the B-B'
section shown in FIG. 1 in the fuel cell system 1 according to a second
exemplary embodiment;

[0052]FIG. 6 is a perspective view depicting the structure of the fuel
tank 8 in the fuel cell system 1 according to the second exemplary
embodiment;

[0053]FIG. 7 is an exploded perspective view depicting the configuration
of the portion positioned between the anode electric collector 24 and the
cathode electric collector 25 in the fuel cell system 1 according to the
second exemplary embodiment;

[0054]FIG. 8A is a top view of a sealing member 15;

[0055]FIG. 8B is a top view of a sealing member 15;

[0056]FIG. 8C is a top view of a sealing member 15;

[0057]FIG. 8D is a top view of a sealing member 15;

[0058]FIG. 9 is an exploded perspective view depicting the configuration
of the portion between the anode electric collector 24 and the cathode
electric collector 25 in the fuel cell system 1 according to a third
exemplary embodiment;

[0059]FIG. 10 is a sectional view of the fuel cell structure on the B-B'
section shown in FIG. 1 in the fuel cell system 1 according to the third
exemplary embodiment;

[0060]FIG. 11 is a schematic view showing the flow path structure, with
regard to the fuel cell systems 1 of Embodiment Examples 1 and 2;

[0061]FIG. 12 is a schematic view showing the flow path structure, with
regard to the fuel cell systems 1 of Comparative Example 1;

[0062]FIG. 13 is measurement results of Embodiment Examples 1 and 2 and
Comparative Example 1; and

[0064]FIG. 1 is a top view of a fuel cell system 1 according to this
exemplary embodiment. The fuel cell system 1 according to the present
exemplary embodiment is provided with: a plurality of fuel cells 5
arranged on a plate-shaped frame 7 so as not to overlap with one another;
a fuel supply flow path 6 provided within the frame 7; and return flow
paths 22 provided within the frame 7 (Also shown in FIG. 1 are the fuel
supply flow path 6 and the return flow paths 22, which are provided
within the frame 7). Used as the fuel is methanol, which is a sort of
liquid fuel.

[0065]The fuel supply flow path 6 is composed of a main flow path 61 and a
plurality of branch flow paths 62 branched from the main flow path 61.
Each branch flow path 62 is branched at the portion of at least one
branch connection 23 from the main flow path 61 and connected to each
fuel cell 5. In this exemplary embodiment, four branch connections 23
(branch connections 23A to 23D) are provided, and two branch flow paths
62 are branched from the main flow path 61 at each branch connection 23;
each branch flow path 62 is connected to the fuel cell 5.

[0066]Here, at three upstream branch connections 23 (the branch
connections 23A to 23C) out of the four branch connections, the sectional
area of the flow path in the downstream is narrower than that in the
upstream. That is, the main flow path 61 has a larger sectional area in
the upstream of the branch connection 23A between the upstream and
downstream of the branch connection 23A. Similarly, the sectional area in
the upstream is larger than the downstream at the branch connections 23B
and 23C. That is, the main flow path 61 has a narrower sectional area as
it goes from the branch connections 23A to 22B, and to 22C. The sectional
areas of the branch flow paths 62 are sufficiently narrowed, compared
with the sectional area of the main flow path 61. It should be noted
that, as to the branch connection 23D, the sectional area in the
downstream is not always required to be narrower; however, it is more
preferable that the sectional area in the downstream is also narrower at
the branch connection 23D, similarly to the branch connections 23A to
23C.

[0067]The fuel that is not consumed in the respective fuel cells 5 are
introduced into the return flow path 22 through return branch flow paths
221 connected to the respective fuel cells 5. The fuel cell system 1 of
the present invention has a planar stack structure with a parallel fuel
supply system for circulating the fuel with such routing.

[0068]FIG. 2 shows a sectional view of the fuel cell system 1 on the B-B'
section of FIG. 1. Each fuel cell 5 is provided with a fuel tank 8 for
holding the fuel received from the branch flow path 62 and an MEA
arranged on the fuel tank 8. The MEAs are each composed of an electrolyte
membrane 4 disposed between an anode 2 and a cathode 3. The MEAs are
arranged such that the anodes 2 are placed to face the fuel tanks 8.
Additionally, frame-shaped anode electric collectors 24 are arranged
between the fuel tanks 8 and the MEAs. Arranged on the cathodes 3 of the
MEAs are frame-shaped cathode electric collectors 25, similarly to the
anode electric collectors 24.

[0069]FIG. 4 is an exploded perspective view showing the structure of the
portions where the anode electric collectors 24, the MEAs, and the
cathode electric collectors 25 are provided. As shown in FIG. 4, the
electrolyte membranes 4 are wider than the anodes 2 and the cathodes 4,
and the edge portions thereof protrude from the portions supported
between the anodes 2 and the cathodes 4. A plurality of frame-shaped
sealing members 15 (15A to 15C) are additionally provided to seal the
MEAs from the outside. The sealing members 15B are arranged on the side
of the anode 2. Preferably, the thickness of the sealing members 15B are
adjusted to the same thickness as that of the anodes 2 so as to avoid the
formation of stepwise structure; this allows sealing the sides of the
anodes 2 from the outside. The sealing members 15C are arranged on the
side of the cathodes 3, similarly to the sealing members 15B. It is also
preferable that the thickness of the sealing members 15C is adjusted to
the same thickness as that of the cathodes 3; this allows sealing the
sides of the cathodes 3 from the outside. Furthermore, the sealing
members 15A are arranged below the anode electric collectors 24 (near the
fuel tanks 8) to seal the gaps between the anode electric collectors 24
and the frame. The sealing members 15A may be provided with arbitrary
thickness. It should be noted that the sealing members 15A are not always
necessary when the anode electric collectors 24 is adhered closely to the
frame 77, and the sealing members 15A may be omitted.

[0070]FIG. 3 is a perspective view showing the structure of the fuel tanks
8. As shown in FIG. 3, the fuel tanks 8 are concaves provided for the
frame 7. Openings 9 are provided through the bottom faces 10 of the fuel
tanks 8. The fuel tanks 8 communicate with the branch flow paths 62
through the openings 9. The openings 9 may be provided through the sides
instead of the bottom faces 10; however, the provision through the bottom
faces 10 is more preferable from the viewpoint of the backflow protection
of the fuel. Preferably, the diameter of the openings 9 ranges from 0.1
to 1.0 mm. Adjusting the diameter of the openings 9 to this range
increases the fuel supply pressure. The increase in the fuel supply
pressure avoids the fuel backflow from the fuel tanks 12 to the fuel
supply flow paths 21.

[0071]The fuel tanks 8 are connected to the return branch flow paths 221
on the opposite sides to the fuel supply flow paths 61 to exhaust the
fuel from the fuel tanks 8 through the branch flow paths 221.

[0072]Wicking members 11 for holding the fuel are inserted into the fuel
tanks 8. The wicking members 11 are intended to absorb and hold the
liquid fuel, mainly by means of capillarity. The insertion of the wicking
members 11 reduces the difference in the flow path resistance among the
respective fuel cells 11, and thereby allows distributing the fuel more
uniformly. Woven cloth, non-woven cloth, fibrous mats, fibrous webs,
foamed plastic and the like may be used as the wicking members 11, for
example, and hydrophilic material, such as hydrophilic urethane foam, and
hydrophilic glass fiber, is preferably used in particular. For the use in
the direct liquid fuel supply in which the anodes 2 are directly placed
over the fuel tanks 8 to supply the liquid fuel to the anodes 2, as
described in this exemplary embodiment, the flow path resistance is
preferably reduced to the degree not to disturb the fuel flow. The use of
material with reduced flow path resistance results in that the fuel is
mainly absorbed into the wicking members 11 and uniformly supplied to the
anodes 2 from the upper portions of the wicking members 11.

[0073]The fuel cells 5 of the present invention, incorporating the cell
structures of the above-described configuration, are screwed and fixed
onto the frame with a plurality of screws (not shown) penetrating the
periphery portions of the cell structures. It should be noted that the
technique for attaching the fuel cells 11 to the frame 10 is not limited
to the screws; other techniques, such as adhesion, may be used instead as
long as the structure avoids the liquid fuel being leaked from the fuel
cells 11.

[0074]It should be noted that polymer films having high proton
conductivity with no electron conductivity are preferably used as the
electrolyte membranes 4 of the MEAs. Ion-exchange resin is suitable for
the constituent material of the electrolyte membranes 4, which has a
polar group, including a strong acid group such as a sulfonic group, a
phosphoric group, a phosphine group, or a weak acid group such as a
carboxyl group; specific examples are a perfluorosulfonic acid type
resin, a sulfonated polyether sulfonic acid type resin, a sulfonated
polyimide type resin. More specifically, the electrolyte membranes 4 may
be solid polymer electrolyte membranes formed of aromatic series polymer
such as sulfonated poly-(4-phenoxy benzoyl-1,4-phenylene), sulfonated
polyether ether ketone, sulfonated polyether sulfone, sulfonated
polysulfone, sulfonated polyimide, alkyl sulfonated polybenzimidazole.
The film thickness of the electrolyte membrane 4 may be properly selected
within the range between about 10 and 300 μm, depending on the
material characteristics, the use field of the fuel cells and so on.

[0075]The cathodes 3 are electrodes that produce water by reducing the
oxygen, as indicated by the Equation (2). For example, the cathodes 3 may
be obtained by depositing a catalyst layer composed of: granules
(including powder) that hold the catalyst on a carrier such as carbon or
the catalyst single body without any carrier; and a proton conductor, on
a substrate such as carbon paper, through coating or the like. The
catalyst may be platinum, rhodium, palladium, iridium, osmium, ruthenium,
rhenium, gold, silver, nickel, cobalt, molybdenum, lanthanum, strontium,
yttrium and the like. The catalyst may be formed of one of materials
listed above or a combination of two of the listed materials. The
granules for carrying the catalyst are exemplified by carbon-based
material, such as acetylene black, ketjen black, carbon nanotube, carbon
nanohorn and so on. When the carbon-based material is particulate, for
example, the size of the granules is properly selected within a range
from about 0.01 to 0.1 μm, more preferably, within a range between
about 0.02 and 0.06 μm. An impregnation method may be applied in order
to make the granules carry the catalyst, for example.

[0076]Solid polymer electrolyte membranes may be used as the substrates on
which the catalyst layers are formed; instead, conductive porous material
may be used, such as carbon paper, carbon molded body, carbon sintered
body, sintered metal, and foam metal. When substrates such as carbon
paper are used, it is preferable that the catalyst layers are formed on
the substrates to obtain the cathodes 31, and then the cathodes 31 are
bonded to the solid polymer electrolyte membranes 33 with a method of a
hot press or the like in a direction in which the catalyst layers are in
contact with the solid polymer electrolyte membranes 33. The catalyst
quantity of the cathodes 31 per unit area may be properly selected within
a range between about 0.1 mg/cm2 and 20 mg/cm2, depending on
the kind and size of the catalyst and so on.

[0077]The anodes 2 are electrodes which produce hydrogen ions, CO2
and electrons from the methanol aqueous solution and the water, and the
anodes 2 are configured similarly to the cathodes 3. The catalyst layers
and substrates of the anodes 2 may be same as those of the cathode 3, or
may be different instead. The catalyst quantity of the anodes 2 per unit
area may be properly selected within a range between about 0.1
mg/cm2 and 20 mg/cm2, depending on the kind and size of the
catalyst and so on, similarly to the case of the cathodes 3.

[0078]The cathode electric collectors 25 and the anode electric collectors
24 are arranged in contact with the cathodes 3 and the anodes 2,
respectively, functioning so as to increase the electron extraction
efficiency and the electron supply efficiency. In this exemplary
embodiment, the electric collectors 24 and 25 are frame-shaped members
provided in contact with peripheral portions of the anodes and the
cathodes. The materials of the electric collectors 24 and 25 may be
stainless steel, sintered metal, foam metal and the like, or members of
the foregoing metal on which high conductive metal material is plated, or
conductors such as carbon material, for example,

[0079]The sealing members (15A to 15C) preferably have the sealing
property, the insulating property or the elastic property, depending on
the necessity. The sealing members 15 may be formed of, for example,
plastic material such as PTFE (polytetrafluoroethylene), PET
(polyethylene terephthalate), PEEK (polyether ether ketone), and vinyl
chloride, or rubber material such as fluorine resin, silicon rubber, and
butyl rubber. The use of the sealing members 15 avoids the fuel being
leaked from the fuel cells 5 to the outside.

[0080]A description is given of the fuel flow in the above-configured fuel
cell system 1 in the following. As indicated by the arrows of FIG. 1, the
fuel firstly flows through the fuel supply flow path 6. Then, the fuel
branched at the branch connections 23 (23A to 23D) are supplied through
the respective branch flow paths 62 to the respective fuel cells 5.

[0081]With reference to FIG. 2, the fuel flowing through the branch flow
paths 62 is supplied from the openings 9 to the fuel tanks 8. The fuel
flowing into the fuel tanks 8 is immersed into the wicking members 11 and
held thereby. The fuel is then immersed out from the upper portions of
the wicking members 11 and supplied to the anodes 2. The fuel supplied to
the anodes 2 causes the reaction indicated by the following Equation 3 on
the anodes, and also causes the reaction indicated by the following
Equation 4 on the cathodes, achieving electric power generation.

CH3OH+H2O→CO2+6H++6e-, (Reaction Equation
3)

and

6H++6e-+ 3/2O2→3H2O. (Reaction Equation 3)

[0082]The electric power generated on the anodes 2 and the cathodes 3 is
taken out by the anode electric collectors 24 and the cathode electric
collectors 25. On the other hand, the fuel that is not supplied to the
anodes 2 is sent from the fuel tanks 8 through the return branch flow
paths 221 to the return flow paths 22 and is exhausted in the direction
of the arrows of FIG. 1.

[0083]As described above, the fuel cell system according to this exemplary
embodiment reduces the inner pressure change inside the supply flow paths
due to the structure in which the sectional area of the fuel flow path in
the downstream of the branch connection is narrower than that in the
upstream thereof. This achieves uniform supply pressures of the
respective fuel cells regardless of the locations from the upstream and
the downstream.

[0084]Additionally, only one-directional flow from the supply flow path to
the fuel tanks is generated due to the fact that the fuel supply to the
respective fuel cells 5 is achieved by ejecting the fuel through the
narrow openings 9 of the diameter between about 0.1 and 1.0 mm from the
bottoms of the fuel tanks 8. This reduces the generation of the backflow
of the fuel, allowing the fuel to be supplied to the respective fuel
cells with more uniform supply pressures.

[0085]As thus described, the liquid fuel with uniform concentrations is
supplied in the uniform state to the respective fuel cells 5 arranged in
parallel, and thereby the respective fuel cells 5 are made uniform in the
electric power generation state, which enables the electric power
generation in the more stable state for a long time.

Second Exemplary Embodiment

(Structure)

[0086]In the fuel cell system 1 in this exemplary embodiment, a plurality
of fuel cells 5 are flatly arranged on the frame 7 similarly to the first
exemplary embodiment. Improvements as compared with the first exemplary
embodiment include: provision of fuel supply control membranes 12 formed
between the fuel tanks 8 and the anodes 2; provision of cover members and
vaporization suppressing members; provision of exhausting portions 13 for
allowing the anodes 2 to communicate with the outside; and provision of
penetrating holes through the wicking members. It should be noted that
descriptions are omitted for the same portions as the first exemplary
embodiment in the following.

[0087]FIG. 5 is a sectional view showing the structure of the fuel cells 5
in the fuel cell system 1 according to this exemplary embodiment. In FIG.
5, the directions of the arrows indicate the directions in which the fuel
easily flows. In each fuel cell 5, the fuel supply control membrane 12 is
arranged on the fuel tank 8, which is the concave of the frame 7. The
anodes 2 of the MEAs are arranged over the fuel supply control membranes
12 to face the fuel supply control membranes 12. The anode electric
collectors 24 are arranged between the anodes 2 and the fuel supply
control membranes 12. Additionally, the sealing members 15A are provided
between the anode electric collectors 24 and the fuel supply control
membranes 12. It should be noted that, although the anodes 2 are in
contact with the fuel supply control membranes 12 on the center portions
thereof in FIG. 5, gaps may exist therebetween. The electrolyte membranes
4 are formed on the anodes 2. The sealing members 15B are arranged on the
sides 14 of the anodes 2, and the sides 14 of the anodes are protected
from the outside. The cathodes 3 are provided on the electrolyte
membranes 4. The frame-shaped cathode electric collectors 25 are arranged
on the cathode 3. The sealing members 15C are also arranged on the sides
of the cathodes 3, and the cathodes 3 are protected from the outside.
Also, vaporization suppressing members 19 are provided to cover the
cathodes 3 and the cathode electric collectors 25. Moreover, cover
members 20 are provided on the vaporization suppressing members 19.

[0088]The fuel tanks 8 are provided as concaves of the frames 7, similarly
to the first exemplary embodiment. FIG. 6 is a perspective view showing
the structure of the fuel tanks 8. In FIG. 6, the direction of the arrow
indicates the direction in which the fuel easily flows. The openings 9
are provided through the bottom faces 10 of the fuel tanks 8. The fuel
tanks 8 are connected to the branch flow paths 62 branched from the main
flow path 61 through the openings 9. The wicking members 11 are inserted
into the fuel tanks B. Penetrating holes (hereafter, referred to as
wicking holes 21) are provided through the wicking members 11 at the
positions corresponding to the openings 9. That is, the openings 9 are
not covered with the wicking members 11 due to the existence of the
wicking holes 21.

[0089]It is preferable that material is used as the wicking members 11,
which allows half or more of the fuel sent to the fuel tanks 8 to flow on
the surface thereof without being absorbed by the wicking members 11. It
is also preferable that the wicking members are filled in the fuel tanks
8 so as to eliminate the spacing therein. It is also preferable that the
wicking holes 21 are provided only in the portions in contact with the
openings 9, having a size equal to or slightly larger than the diameter
of the openings 9. Such structure facilitates ejection of the fuel
supplied from the openings 9 in the lateral direction of the anodes 2. As
a result, the fuel is selectively sent toward the anodes 2. Also, the use
of material which allows half or more of the fuel sent to the fuel tanks
8 to flow on the surface without being absorbed in the wicking member 11
results in that the fuel supplied from the bottoms is sent to the upper
portions of the wicking members 11 and guided to flow on the upper
surfaces of the wicking members 11, as indicated by the arrows in FIGS. 5
and 6. This enhances the effect of selectively sending the fuel to the
anodes 2 through the fuel supply control membranes 12. In other words,
only one-way fuel flow to the MEA is generated, thereby enabling more
uniform distribution of the fuel to the respective fuel cells. This
avoids the lack of the fuel supply to the anodes 2 in the high current
condition, thereby keeping more uniform electric power generation state
with high output. When the fuel flowing on the surfaces of the wicking
members 11 is half or less of the fuel sent to the fuel tanks 8, on the
other hand, this may cause the fuel to be mainly held by the wicking
members 11, resulting in insufficient amount of the fuel supplied to the
anodes 2 through the fuel supply control membranes 12.

[0090]It should be noted that it is preferable that the fuel supply
control membranes 12 and the fuel tanks 8 are positioned closely to each
other across gaps for flowing the liquid fuel therebetween, so that the
liquid fuel flowing on the upper surfaces of the wicking members 11 and
the liquid fuel once held by the wicking members 11 are directly supplied
to the fuel supply control membranes 12 from the wicking members 11.

[0091]The fuel supply control membranes 12 are control membranes which
allow selectively transmitting only the gas component of the liquid fuel.
The fuel supply control membranes limit the supply amount of the fuel to
the anodes 2. As a result, the optimal amount of the fuel is always
supplied to the anodes 2 to maintain stable electric power generation.

[0092]The fuel supply control membranes 12 are fixed to the upper
apertures of the fuel tanks 8, into which the wicking members 11 having
the fuel holding ability are inserted. The fuel supply control membranes
12 are in contact with or positioned close to the electrodes configuring
the anodes 2 by the pressure caused by the fuel supplied to the fuel
tanks 8. Hydrophobic gas liquid separation membranes such as PTFE
(polytetrafluoroethylene) porous body are mainly used as the fuel supply
control membranes 12. The fuel supply amount to the fuel supply control
membranes 12 is required at least to be equal to or greater than the
consumption amount of the fuel in the MEAs. The fuel supply quantity to
the fuel supply control membranes 12 is determined depending on: the
properties resulting from the material characteristics of the fuel supply
control membranes 12, such as the differences in the film thickness and
the porosity; and the fuel transmission efficiency resulting from the
external factors, such as the temperature and the humidity. It should be
noted that the fuel supply control membranes 12 are not always required
to be hydrophobic when exhausting portions 13 are additionally provided;
and the hydrophilic porous membranes and the like may be used instead.

[0093]A description is then given of the exhausting portions 13 in the
following. Each fuel cell 5 includes an exhausting portion 13 for
exhausting CO2 produced on the anode 2 to outside. FIG. 7 is an
exploded perspective view illustrating the structures of the anode
electric collectors 24, the MEAs and the cathode electric collectors 25
in the fuel cells 5. Similarly to the first exemplary embodiment, the
sealing members 15B and 15C are provided to seal the sides of the anodes
2 and the cathodes 3. Also, the sealing members 15A are provided below
the anode electric collectors 24. It should be noted that exhausting
paths 16 are provided as the exhausting portions 13 through the sealing
members 15B.

[0094]FIG. 8B is a top view of the sealing members 15B. Provided for at
least one edge of the sealing members 15B are the exhausting paths 16.
The exhausting paths 16 may be provided by forming the sealing members
15B as a combined structure of small piece members. In this case, the
gaps between the members are used as the exhausting paths 16. It should
be noted that the shape of the exhausting paths 16 are not limited to the
above-described shape. Concaves or slits may be formed through the
sealing member 15B, and the concaves may be used as the exhausting paths
16. As described above, the exhausting paths 16 allow exhausting CO2
produced on the anodes 2 to the outside through the exhausting paths 16.

[0095]It should be noted that the sealing members 15A and 15B are
structured without any gap as shown in FIG. 8A. Also, screw holes 26 are
formed through the sealing members 15A, 15B and 15C. The sealing members
15 are fixed to the frame 7 with the screws inserted into the screw holes
26. It should be noted that the respective sealing members 15 preferably
have the sealing property, the insulating property and/or the elastic
property, depending on the necessary, and the sealing members 15 are
usually made of rubber and plastic which have the sealing ability. The
sealing members 15A and 15C are desired to have the sealing ability to
avoid the fuel leakage and the like.

[0096]The sealing members 15 may be each made of plastic material such as
PTFE (polytetrafluoroethylene), PET (polyethylene terephthalate), PEEK
(polyether ether ketone), and the vinyl chloride; or rubber material such
as fluorine resin, silicon rubber, and butyl rubber. The sealing members
15B are not always required to have the sealing ability, and therefore
washer-shaped members or meshed members may be inserted as the spacers
when structured to allow uniform electric collection.

[0097]The exhausting paths 16 may be formed for only one edge of the
frames, or may be formed for two opposed edges, or may be formed for the
four edges, while the shapes and arrangements thereof are not required to
be equal among the respective fuel cells 5. Also, the number and size of
the exhausting paths 16 are not especially limited; however, the number
and size are preferably selected so that at least CO2 is effectively
exhausted.

[0098]A description is then given of the vaporization suppressing members
19 in the following with reference to FIG. 5. The vaporization
suppressing members 19 are arranged on the cathodes 3 and the cathode
electric collectors 25 so as to cover the cathodes 3 and the cathode
electric collectors 25. As for a planar type fuel cell system, such as
the fuel cell system of this exemplary embodiment, the whole structure is
typically enclosed by a housing, and this may necessitate forced air blow
toward the cathodes 3. When the fuel supply control membranes 12 are
provided near the anodes 2, the electrolyte membranes 4 are not in direct
contact with the liquid fuel, and this results in the decrease in the
water concentration of the electrolyte membranes 4. Moreover, when the
sides of the cathodes 4 are subjected to air blow, the water
concentration of the cathodes 4 is also extremely decreased. The
vaporization suppressing members 19 are provided in order to suppress the
water vaporization caused by the air blow mentioned above and to thereby
maintain the humidity condition suitable for the electric power
generation.

[0099]The vaporization suppressing members 19 are members having the
moisturizing property, for which member cellulose fiber and the like are
used as raw materials. Hydrophilic materials such as woven cloth,
non-woven cloth, fibrous mats, fibrous webs, and cellular plastic are
exemplified as the vaporization suppressing member 19. It should be noted
that the introduction of air necessary for the electric power generation
can be enhanced by using the structure in which the air is introduced
from the sides of the covers, or the structure in which holes are
provided through the vaporization suppressing member 19, when the
vaporization suppressing members 19 are used as the covers; however, the
mechanism for enhancing the air introduction is not always necessary,
since the vaporization suppressing members 19 themselves have the air
permeability. Preferably, the vaporization suppressing members 19 and the
cathodes 4 are in contact with each other; desirably-structured
supporting members or spacers may be used to separate the cathodes 4 from
the vaporization suppressing members 19.

[0100]The cover members 20 are provided to cover the vaporization
suppressing members 19. Holed plates or meshed members of metal or
plastic are suitably used as the cover members 20; for example, punching
sheets of SUS having a surface coated with insulating paint are
preferably used. When the cover members 19 are used, the vaporization
suppression layers may be omitted under the small current and weak
ventilation conditions, since the cover members 19 themselves have the
effect of suppressing the waver vaporization; however, the insertion of
the vaporization suppressing layers 51 is necessary when the electric
power generation is executed under the high current density conditions of
100 A/cm2 or more or the strong air blow conditions. With regard to
openings of the cover members 20, small circular holes and the like are
desirably provided such that the opening ratio ranges between about 5 and
50%.

[0101]In this exemplary embodiment, the following advantageous effects are
achieved in addition to the advantageous effects of the first exemplary
embodiment:

[0102]The fuel supply with the fuel supply control membranes 12, as in the
fuel cell system 1 according to this exemplary embodiment, potentially
causes poor fuel circulation, due to the insertion of the wicking members
11 into the fuel cells, which causes the increase in the flow path
resistance for the liquid holding; however, the structure for ejecting
the fuel from the wicking holes 21 towards the anodes 2 through the
openings 9 provided for the bottom faces 10 of the fuel tanks 8 enhances
the selective fuel flow on the surfaces of the wicking members 11, which
are opposed to the anodes 2. As a result, the fuel is rather efficiently
supplied to the anodes 2. Moreover, this suppresses the unbalance among
the inner pressures of the respective fuel cells 5 caused by the
accumulation of CO2 inside the fuel cells 5, and the backflow of the
CO2 and fuel to the fuel supply flow path 6 caused by the increase
in the inner pressures.

[0103]The CO2 produced on the anodes 2 during the electric power
generation is exhausted outside the fuel cells 5 from the exhausting
paths 16 provided for the sealing members 15B through the gaps formed
through the electrodes configuring the anodes 2 or between the anodes 2
and the fuel supply control membranes 12. Such CO2 exhaustion
mechanism allows the CO2 to be exhausted from the neighbor of the
anodes 2 while the fuel is vaporously supplied to the anodes 2. This
avoids the CO2 produced on the anodes 2 staying between the anodes 2
and the fuel supply control membranes 12.

[0104]Additionally, the exhaustion of the CO2 to the outside from the
exhausting paths 16 formed through the sealing members 15B prevents the
pressure of the regions between the anodes 2 and the fuel supply control
membranes 12 from exceeding the pressure of the regions between the fuel
tanks 8 and the fuel supply control membranes 12. This avoids the gas
flow from the anodes 2 to the fuel tanks 8 through the fuel supply
control membranes 12, allowing the sufficiently higher fuel supply
pressure of the anodes 2. As a result, the reaction efficiency on the
anodes 2 is increased, while stable electric power generation is achieved
for a long time even under the high current conditions.

[0105]Also, the fuel is uniformly distributed to the respective fuel cells
5 irrespectively of the use of the parallel flow path structure, since
the variations in the supply pressure caused by the back pressure
distribution generated in the return flow path 22 in association with the
CO2 generation are also reduced.

[0106]Moreover, the present exemplary embodiment does not require complex
structure; the structure can be extensively simplified. Additionally, the
present exemplary embodiment is superior in cost and safety, since the
leakage of the liquid fuel is avoided by the fuel supply control
membranes 12. In addition, the fuel vaporization through the electrolyte
membranes 4 and the cathodes 3 is suppressed by the vaporization
suppressing members 19 and the cover members 20, and therefore the fuel
is not wastefully consumed as compared with the structure in which the
liquid fuel is directly supplied; this results in the increase in the
electric power generation efficiency.

[0107]Furthermore, the present exemplary embodiment decreases the water
vaporization from the cathodes and thereby achieves the optimal humidity
condition for the electric power generation, since the surfaces of the
cathodes 3 are covered by the vaporization suppressing members 19 made of
moisturizing material, and the cover members 20 formed of the holed
plates made of metal and the like on the vaporization suppressing members
19. As a result, the wasteful vaporization of the fuel is reduced,
thereby improving the electric power generation efficiency.

Third Exemplary Embodiment

[0108]A description is given of a fuel cell system 1 according to the
third exemplary embodiment of the present invention in the following. The
fuel cell system 1 of this exemplary embodiment differs from that of the
second exemplary embodiment in the structure of the exhausting portions
13. It should be noted that the portions other than the exhausting
portions 13 are similar to those in the second exemplary embodiment, and
therefore the explanations thereof are omitted.

[0109]FIG. 9 is an exploded perspective view depicting the stack structure
of the MEAs, the anode electric collectors and the cathode electric
collectors 25 in the fuel cell system 1 according to this exemplary
embodiment. In FIG. 9, the arrow direction indicates the direction in
which the CO2 is exhausted. The sealing members 15A to 15C are
arranged similarly to the second exemplary embodiment. FIG. 8C is a plan
view of the sealing members 15C, and FIG. 8D is a plan view of the
sealing members 15B. As shown in FIG. 5D, two notches 17B extending
inward are provided for the sealing members 15B. On the other hand, as
shown in FIG. 50, notches 17C extending outward are provided for the
sealing members 15C. Additionally, as shown in FIG. 9, holes 18 are
provided through the electrolyte membranes 4 on the notches 17C. FIG. 10
is a sectional view depicting the stack structure from the anode electric
collectors 24 to the cover members 20. The exhausting paths 16 are
composed of the above-mentioned notches of the sealing members 15B, 15C
and holes provided through the electrolyte membranes 4' Through these
exhausting paths 16, the anodes 2 communicate with the outside. It should
be noted that the holes 18 provided through the electrolyte membranes are
preferably have a diameter between about 0.3 and 2.0 mm. The diameter in
this range allows effective exhaustion of CO2.

[0110]The present exemplary embodiment offers the following advantageous
effect in addition to the advantageous effects in the second exemplary
embodiment:

[0111]In this exemplary embodiment, the CO2 produced on the anodes 2
is exhausted from the anodes 2 to the outside through the notches 17B,
the holes 18 and the notches 17C. This avoids accumulation of the
CO2 in the fuel cells 5, preventing the gas from being introduced
into the fuel circulating system.

[0112]In the following, a specific description is given of the fuel cell
system of the present invention, depicting examples.

Embodiment Example 1

[0113]The structure of the fuel cell used in Embodiment Example 1 is
described below. At first, the catalyst-carrying carbon granules were
prepared, which are composed of carbon granules (Ketjen Black EC600JD
manufactured by Lion Corporation) carrying platinum micro granules having
a granule diameter in a range from 3 to 5 nm with the weight ratio of
50%, and catalyst paste for cathode formation was obtained by adding 5
weight % Nafion solution manufactured by E.I. du Pont de Nemours and
Company (Article Name; DE521, "Nafion" is the registered trademark of
E.I. du Pont de Nemours and Company) to 1 g of the catalyst-carrying
carbon granules, followed by agitation. Cathodes 3 of 4 cm×4 cm are
then formed through coating this catalyst paste on carbon paper
(TGP-H-120 manufactured by Toray Industries, Inc.) as substrates with the
coating quantity of 1 to 8 mg/cm2, followed by drying. Anodes 2 were
also prepared similarly to the cathodes. It should be noted, however,
that platinum (Pt)-ruthenium (Ru) alloy granules (the Ru concentration is
50 at %) having a granule diameter in a range from 3 to 5 nm were used
instead of the platinum granules for the preparation of anodes 2.

[0114]Membranes of 8 cm×8 cm with a thickness of 180 μm made of
Nafion 117 (the number average molecular weight thereof is 250000)
manufacture by E.I. du Pont de Nemours and Company were then prepared as
the electrolyte membranes 4. The cathodes 3 were each arranged on one
face facing the thickness direction of the electrolyte membrane 4, in the
direction in which the carbon paper was positioned outward. The anodes 2
were each arranged on the other face in the direction in which the carbon
paper was positioned outward. This was followed by hot pressing from the
outer sides of the respective sheets of carbon paper. Consequently, the
cathodes 3 and the anodes 2 were bonded to the electrolyte membranes 4 to
complete MEAs (Membrane Electrode Assembly).

[0115]This was followed by placing anode electric collectors 24 and
cathode electric collectors 25 on the cathodes 3 and the anodes 2, each
of which collectors was formed of a rectangular frame plate of stainless
steel (SUS316) of a thickness 200 μm, having an outer dimension of 6
cm2, a thickness of 1 mm, and a width of 11 mm. Also, rectangular
silicon rubber frames having an outer dimension of 6 cm2, a
thickness of 0.3 mm and a width of 10 mm were arranged between the
electrolyte membranes 4 and the anode electric collectors 24 to function
as the sealing member 15B. Here, two notches of a width of 0.5 mm were
provided as the exhausting paths 16 for each frame edge of the sealing
members 15B. As for other sealing members 15, arranged were sealing
members 15A and 15C each formed of a rectangular frame plate of silicon
rubber having an outer dimension of 6 cm2, a thickness of 0.3 mm,
and a width of 10 mm.

[0116]A frame of PP (poly propylene) having an outer dimension of 15
cm×30 cm with a thickness of 1 cm was used as the frame 7 of the
fuel cell system 1, and eight concaves were formed as the fuel tanks 8 so
as to accommodate fuel cells 1 in two columns and four rows within the
frame 7 (See FIG. 11). Provided at the center of the frame 7 was a main
flow path 61 of the fuel supply flow path 6 which communicated with the
respective fuel tanks 8 through branch flow paths 621 to 624. It should
be noted that the branch connections 23 are denoted by symbols A to D
attached thereto for identification. The main flow path 61 was
sequentially decreased in the sectional area to 20 mm2, 15 mm2
and 10 mm2 as the fuel is branched to the branch flow paths 621
(provided at the branch connection 23A), to the branch flow paths 622
(provided at the branch connection 23B) and to the branch flow paths 633
(provided at the branch connection 23C). It should be noted that the
sectional area of the main flow path 61 from the inlet to the branch
connection 23A was adjusted to 25 mm2. The size of the fuel tanks 8
was adjusted to have a height of 8 mm, an aperture of 4×4 mm and a
depth of 3 mm. The openings 9 for the fuel supply were provided through
the bottom faces to communicate with the branch flow paths 621 to 624.
The wicking members 11 of urethane material serving as fuel holding
material were inserted into the fuel tanks 8. Also, the fuel tanks 8 were
connected to communicate with the two return flow paths 22 through the
return branch flow path 221.

[0117]The MEAs, the cathode electric collectors 25, the anode electric
collectors 24, the fuel supply control membranes 12 and the sealing
members 15 (15A to 15C) were screwed with a predetermined number of the
screws and integrated to complete fuel cells 11 of Embodiment Example 1.

[0118]Eight fuel cells 11 prepared as mentioned above were arrayed and
fixed to the frame 7, which has the fuel flow path structure shown in
FIG. 11. The adjacent cells were all connected electrically in series
through the electric collectors. In FIG. 11, a negative terminal was
drawn from the fuel cell at the position denoted by the symbol "5", and a
positive terminal was drawn from the fuel cell at the position denoted by
the symbol "6".

Embodiment Example 2

[0119]A description is given of the structure of the fuel cells used in
Embodiment Example 2 in the following. The manufacturing method and
structure of the MEAs are similar to those of Embodiment Example 1. The
flow path structure of Embodiment Example 2 was identically structured to
Embodiment Example 1 (See FIG. 11). That is, the fuel supply flow path
was decreased in the sectional area in the downstream as it passed
through the branch connections.

[0120]Added to Embodiment Example 2 were fuel supply control membranes 12.
Used as the fuel supply control membrane 12 were PTFE porous membranes of
8 cm×8 cm with a thickness of 50 μm (having a porous diameter of
1.0 μm and a porous ratio of 80%). Additionally, cotton fiber mats of
4 cm×4 cm were placed as the vaporization suppressing members 19
(the moisturizing layers) on the cathodes 3, and punching sheets of SUS
having a thickness of 0.5 mm, a hole diameter of 0.75 mm and an opening
ratio of 20% were placed as the cover members 20 thereon to fix the
vaporization suppressing members 19.

[0121]The fuel cells 5 of Embodiment Example 2 were formed as a structure
in which the MEAs, the cathode electric collectors 25, the anode electric
collectors 24, the fuel supply control membranes 12, the sealing members
15, and the sealing members 15B with the exhausting paths 16 were screwed
with a predetermined number of the screws to be integrated, and the
vaporization suppressing members 19 and the cover members 20 were
attached thereto. The wicking members 11 of the urethane material were
inserted as the fuel holding material into the fuel tanks 8. Here, used
as the urethane material was a type of material designed to increase the
flow path resistance by fluid replacement, allowing the fuel to flow more
easily on the surface than inside the wicking members 11. Additionally,
semicircular small holes having a diameter of 0.7 mm were provided at the
upper portions corresponding to the openings 9 of the wicking members 11,
used as the wicking holes 21. It should be noted that the fuel cells 5
were structured as shown in FIG. 5. Similarly to Embodiment Example 1,
adjacent cells were all electrically connected in series by the electric
collectors. In FIG. 11, the negative terminal was drawn from the fuel
cell at the position denoted by the symbol "5", and the positive terminal
was drawn from the fuel cell at the position denoted by the symbol "6".

Comparative Example 1

[0122]A description is given of the structure of the fuel cell used in
Comparison Example 1 in the following. The manufacturing method and
structure of the MEAs are similar to that of Embodiment Example 1. The
flow path structure was structured so that the fuel was branched into the
respective electric power generators from the main flow path 61 of the
fuel supply flow path 6 positioned at the center and sent to the return
flow paths 22 on the both ends, as shown in FIG. 12. Differently from
Embodiment Examples 1 and 2, the main flow path 61 had a constant
sectional area of 20 mm2 between the fuel branch flow paths 621 and
624. The structure of the fuel cells 5 within the respective electric
power generators were structured as shown in FIG. 5, and relatively rough
urethane foam having a low flow path resistance was inserted as the
wicking members 11. Adjacent cells were all electrically connected in
series by the electric collectors. In FIG. 12, a negative terminal was
drawn from the fuel cell at the position denoted by the symbol "5", and a
positive terminal was drawn from the fuel cell at the position denoted by
the symbol "6".

(Experimental Result)

[0123]FIG. 13 shows the surface temperatures and the voltages of the
cathodes 2 of the fuel cell systems of Embodiment Examples 1 and 2 and
Comparison Example 1, obtained by implementing an electric power
generation test for 30 minutes under the conditions in which 200 mL of 10
vol % methanol aqueous solution is circulated and supplied with a flow
velocity of 10 mL/min in an atmosphere environment of a temperature of
25° C. and a humidity of 50% with a current level corresponding to
the current density of 100 mA/cm2. In the actual experiment, only
the fuel cells 5 denoted by the numerals in FIG. 13 were monitored.

[0124]A portion of Comparison Example 1 experiences low voltage and the
cathode surface temperature in this portion is higher by 10° C. or
more as compared with other portions, due to the difficulty in supplying
the fuel to a specific particular fuel cell 5. When the sectional area of
the fuel supply flow path 6 is constant, the inner pressure is decreased
as the fuel is branched, and therefore the inner pressure of the fuel
supply flow path 6 is not sufficiently high as compared with the inner
pressure of the fuel tanks 8, which is increased by the CO2 produced
by electric power generation, at the stage at which the fuel arrives at
the branch flow paths in the downstream of the fuel supply flow path 6.
This prohibits the smooth flow of the fuel due to the decreased fuel
supply pressure to the fuel tanks 8. Additionally, the unsmooth supply of
the fuel to the fuel cell 5 hinders the cooling of the MEAs by the fuel,
and causes high temperatures of the MEAs within the fuel cells 5 when the
current is continuously and forcedly drawn in such conditions; this
causes control difficulty. Such portion in which the fuel supply is
insufficient does not always exhibit only in the fuel cells 5 at the most
downstream portion; such portion may exhibit even in the fuel cells 5 at
the upstream portion or the middle portion, depending on the CO2
production state and exhaustion system from the fuel tanks 8.

[0125]On the other hand, the backflow of the fuel to the fuel supply flow
path 6 is suppressed in Embodiment Example 1, since the inner pressure
drop in the fuel supply flow path 6 caused by the fuel branching is
reduced by the structure in which the sectional area of the fuel supply
flow path 6 is sequentially narrowed, and since the fuel supply pressure
to the fuel tanks 8 is increased by the reduced aperture of the openings
9. This achieves uniform fuel supply to the respective fuel cells 5,
resulting in the reduction in the cathode surface temperature of the MEAs
down to about 7° C. at a maximum and also the reduction in the
voltage distribution. It should be noted, however, that Embodiment
Example 1 may cause local and/or temporal variations in the electric
power generation state, since the inner pressure increase in the fuel
tanks 8 caused by the CO2 produced in the electric power generation
is not improved.

[0126]Furthermore, Embodiment Example 2, which employs the approach in
which the fuel is vaporously supplied from the fuel supply control
membranes 12 and the CO2 produced in the electric power generation
is exhausted from the exhausting portions 13 provided through the sealing
members 15B, allows substantially no CO2 to be introduced into the
fuel tanks 8 and the return flow paths 22, reducing the variations in the
fuel supply pressure to the respective fuel cells 5 due to the back
pressure caused by the CO2 production and the voltage distribution
of the respective fuel cells 5, while making the surface temperature of
the cathodes 3 approximately uniform.

[0127]FIG. 14 shows temporal changes of the voltages for Embodiment
Examples 1 and 2 and Comparative Example 1, when the electric power
generation is executed for about 60 minutes under the same conditions as
the above-described experiment. Comparative Example 1 exhibits a low
original voltage, and suffers from a decrease in the voltage with time.
On the contrary, Embodiment Example 1 stably achieves a high voltage, and
does not exhibit a tendency of the voltage decrease with time.
Furthermore, Embodiment Example 2 stably achieves a higher voltage, and
never exhibit a tendency of the voltage decreased with time.

[0128]As thus described, the use of the approaches of the present
invention exemplified by Embodiment Examples 1 and 2 improves the
respective pressure balances in the fuel supply flow path 6, the return
flow paths 22 and the fuel tanks 8, which have been a problem for the
parallel fuel flow path structure, consequently achieving supplying the
fuel with uniform concentration to the respective fuel cells 5 under the
substantially same conditions. This consequently achieves uniform
electric power generation states in the respective fuel cells 5 and
long-time stabilization of the electric power generation over the entire
fuel cell system 1.